In the art of electrostatography, image patterns of electrostatic charge are formed on an insulating surface by a variety of well-known methods. For example, in photoconductive methods electrostatic charge is caused to dissipate imagewise from the surface of a charged photoconductive layer toward a grounded electrode layer by exposing the layer to actinic radiation. The resulting electrostatic charge pattern is subsequently developed--or transferred to another element and developed--by contact with a developer composition.
A commonly employed developer composition comprises a dry, two-component blend of toner particles and carrier particles. The particles in such compositions are selected and formulated so that the toner particles acquire a charge of one polarity and the carrier particles acquire a charge of opposite polarity by triboelectrification. To this end, it has become common practice to coat the surface of the carrier particles with an appropriate polymer to impart by triboelectrification an electrostatic charge of the desired polarity and magnitude on the toner particles which rub against and cling to the carrier particles.
For certain applications, it is highly desirable to have a negatively charging carrier to provide a positively charged electrographic toner or to strengthen the positive charge in a weakly positive developer. (The polarity of charge on the toner is the polarity assigned to the developer so that, for example, a positively charged developer is one in which the toner particles are charged positively and in which the carrier charges negatively.)
Perfluorinated electronegative polymers have been used as carrier coatings in the art to obtain positive charging developers. However, the carriers coated with fluoropolymers are difficult to prepare, are expensive and limited in choice, and do not present as high a charge as is sometimes desired.
U.S. Pat. No. 3,672,928 discloses a carrier comprising a core coated with a crosslinked unsaturated polyester prepared from allyl esters including diallyl chlorendate. The present inventors have found in connection with their efforts to provide high negatively charging carriers that polymer coatings containing chlorendate groups in which both dicarboxylate functions are blocked (such as by divalent incorporation into the polymer backbone as in U.S. Pat. No. 3,672,928 or by reaction with cross-linking agents) do not provide sufficiently high negative charging. U.S. Pat. No. 3,672,928 further discloses that, although carriers coated with such polymers have a positive triboelectric value, they may be converted into reversal carriers by incorporating small amounts of reversal dyes into the polymer coating. In this regard, the present invention avoids the use of additives in the polymer coating on the carrier and still provides high-charging carriers of negative polarity.